A gap plasmon is an electromagnetic wave propagating in a gap between two noble metal surfaces. Such gap plasmons have previously been studied using only a classical description of the noble metals, but this model fails and shows unphysical behavior for sub-nanometer gaps. To overcome this problem quantum spill-out is included in this paper by applying Density-Functional Theory (DFT), such that the electron density is smooth across the interfaces between metal and air. The mode index of a gap plasmon propagating in the gap between the two metal surfaces is calculated from the smooth electron density, and in the limit of vanishing gap width the mode index is found to converge properly to the refractive index of bulk metal. When neglecting quantum spill out in this limit the mode index shows unphysical behavior and diverges instead.
The mode index is applied to calculate the reflectance of an ultrasharp groove array in silver, as gaps of a few nm are found in the bottom of such grooves. At these positions the gap plasmon field is highly delocalized implying that it mostly exists in the bulk silver region where absorption takes place. Surprisingly, when the bottom width
is a few nm and the effect of spill out at a first glance seems to be negligible, strong absorption is found to take place 1-2 Å from the groove walls as a consequence of the dielectric function being almost zero at these positions. Hence quantum spill out is found to significantly lower the reflectance of such groove arrays in silver.
This paper theoretically studies how the optics of multiple grooves in a metal change as the number of grooves gradually increased from a single groove to infinitely many arranged in a periodic array. In the case of a single groove, the out-of-plane scattering (OUP) cross section at resonance can significantly exceed the groove width. On the other hand, a periodic array of identical grooves behaves radically different and is a near-perfect absorber at the same wavelength. When illuminating multiple grooves with a plane wave, the OUP cross section is found to scale roughly linearly with the number of grooves and is comparable with the physical array width even for widths of many wavelengths. The normalized OUP cross section per groove even exceeds that of a single groove, which is explained as a consequence of surface plasmon polaritons generated at one groove being scattered out of the plane by other grooves. In the case of illuminating instead with a Gaussian beam and observing the limit as the incident beam narrows and is confined within the multiple-groove array, it is found that the total reflectance becomes very low and that there is practically no OUP. The well-known result for periodic arrays is thus recovered. All calculations were carried out using Green’s function surface integral equation methods taking advantage of the periodic nature of the structures. Both rectangular and tapered grooves are considered.
This paper studies theoretically how the optics of multiple grooves in a metal change as the number of grooves is increased gradually from a single groove to infinitely many arranged in a periodic array. In the case of a single groove the out-of-plane scattering (OUP) cross section at resonance can significantly exceed the groove width. On the other hand a periodic array of identical grooves behaves radically different and is a near-perfect absorber at the same wavelength. When illuminating multiple grooves with a plane wave the OUP cross section is found to scale roughly linearly with the number of grooves and is comparable to the physical array width even for widths of many wavelengths. The normalized OUP cross section per groove even exceeds that of a single groove, which is explained as a consequence of surface plasmon polaritons generated at one groove being scattered out-of-the-plane by other grooves. In the case of illuminating instead with a Gaussian beam, and observing the limit as the incident beam narrows and is confined within the multiple-groove array, it is found that the total reflectance becomes very low and that there is practically no out-of-plane scattering. The well-known result for periodic arrays is thus recovered. All calculations were carried out using Greens function surface integral equation methods taking advantage of the periodic nature of the structures. Both rectangular and tapered grooves are considered.
This paper studies theoretically light trapping in a solar cell configuration consisting of a 50-500 nanometer-thin planar silicon (aSi:H) film with a planar silver back-reflector, and scatterer(s) placed directly on the silicon surface. The usual picture for thicker films is that part of the light incident on the scatterer(s) can be coupled into the silicon film at a continuum of angles above the critical angle for the silicon-air interface, in which case light will be trapped and subsequently absorbed. However, for thin films a more appropriate picture is that of light being coupled into the guided modes of the air-silicon-silver geometry corresponding to discrete angles. The scattering of light into each guided mode, and out-of-plane scattering, will be quantified by the related scattering cross section. It will be shown that scatteringcross- section spectra have sharp resonances near cut-off wavelengths of guided modes, with more closely spaced resonances for thicker films. Total resonant cross sections can easily exceed physical cross sections by a factor 10. This study also includes light trapping due to coupling into the Surface-Plasmon-Polariton mode that exists due to the silver surface. It will be shown that peaks in scattering cross sections can be tuned via the geometry to the appropriate wavelength range where light trapping is advantageous due to weak absorption in the silicon, resulting in an optimum thickness around 250 nanometers. We consider both theoretical calculations with and without material losses, and both dielectric and metal scatterers are considered. The calculations were carried out with Green’s function integral equation methods.
This paper considers a range of plasmonic-black-metal polarizers suitable for ultra-short pulses. The polarizers consist of
a metal surface being nanostructured with a periodic array of ultra-sharp grooves with periods of 250-350 nanometers,
and groove depths around 500 nanometers. The surfaces can be designed such that practically all incident light with
electric field perpendicular to the groove direction is absorbed. The efficient absorption is due to incident light being
coupled into gap-plasmon polaritons that propagate downwards in the gaps between groove walls towards the groove
bottom, where it is then subsequently absorbed during propagation. Reflection is largely avoided due to an adiabatic
groove taper design. The other polarization, however, is very efficiently reflected, and the main point of this paper is that
the reflection is with negligible dispersive stretching even for ultra-short pulses of 5-10 femtoseconds temporal width in
the visible and near-infrared. Temporal pulse shapes after reflection are calculated by decomposing the incident laser
pulse into its Fourier components, multiplying with the reflection coefficient in the frequency domain, and then Fouriertransforming
the product back to the time-domain. Reflection of pulses is compared for polarizers based on different
metals (gold, nickel, chromium). Polarizers are studied for two groove-array designs and two directions of light
incidence, center wavelengths 650 nm and 800 nm, and pulse widths 5 fs and 10 fs for the incident pulse.
Optical resonances in a single triangular-shaped metal groove and a periodic array of grooves are studied theoretically
with the Green's function surface integral equation method. In the case of a single groove we study the geometric
resonances for different groove heights, and show that the groove resonances can be explained by standing waves in the
gap being reflected at both the closed groove bottom and the open groove top. We also present the reflection that will be
obtained for different cases of picking up the reflected light within a small or large angular range. Large resonant fields
at the groove bottom are explained as being due to nanofocusing by the groove which can also be thought of as a closed
tapered gap. In the case of a periodic array of grooves we find that resonances of individual grooves are still present in
near-field enhancement spectra and reflection spectra but there are also e.g. very sharp resonances (Rayleigh anomalies)
at wavelengths near the cutoff wavelength of higher grating-reflection orders. Typical resonant enhancements can easily
be two times higher compared with the case of a single groove. The resonances can be realized in the wavelength range
from the visible to the infrared by varying groove height, angle, and periodicity.
A theoretical electrostatic approach for determination of plasmon eigenresonances and absorption cross section
spectra of arbitrarily shaped metal nanoparticles with cylindrical symmetry in stratified geometries is presented.
The method is based on a surface integral equation for the surface polarization charge density. From symmetry
considerations and by incorporating all effects of the stratified surrounding into the Green's function we show
how the three dimensional analysis can be reduced to a single integral over the polar angle along the surface of the
metal nanoparticle. The theoretical scheme is exemplified by analyzing silver nanoparticles shaped as spheres,
oblate spheroids, and nanodisks in different surroundings involving silicon. The effect of varying the distance
between a silver sphere and a silicon surface on plasmonic eigenvalues and absorption cross section spectra is
presented. By flattening silver oblate spheroids and nanodisks embedded in a homogenous silicon surrounding
it is shown how the fundamental horizontally polarized plasmon resonance can be shifted into the near infrared
wavelength range. Also the effect of varying the thickness of thin silicon films with silver nanoparticles embedded
is presented. The results indicate that silver nanoparticles embedded in silicon could be interesting for plasmon
assisted solar cells.
Scattering resonances of metal nano-strip resonators are described as a consequence of formation of standing waves due
to counter-propagating short-range (and slow) surface plasmon polaritons and gap plasmon polaritons, which are
electromagnetic waves bound to and propagating along a nanometer-thin metal film, and a nanometer-sized gap between
metal surfaces, respectively. Scattering spectra and resonant fields are presented for single-metal-nano-strip resonators
and gap plasmon resonators with two closely spaced metal nano-strips. It is shown how strip resonators can be designed
for any resonance wavelength in the range from 600nm to 1600nm.
Surface-plasmon-polariton (SPP) resonators consisting of metal strips in free space, and gap plasmon polariton
resonators consisting of a metal strip close to either a block of metal or a metal surface, are studied as optical
resonators. The analysis is performed using the Green's function surface integral equation method. For strips
in free space, we show how the scattering resonances can be understood, by thinking of the strips as optical
resonators for short-range SPPs. The two gap resonator configurations, strip-block and strip-surface, have
different structure terminations as the width of the strip and the block are identical whereas the surface is
infinite. In the strip-surface configuration, the scattering resonances are broader and red-shifted, compared to
the strip-block configuration. This is explained as a consequence of the effective length of the resonator being
larger in the strip-surface configuration. By varying the gap size, we study the transition from a SPP resonator
to a gap plasmon polariton resonator. In the strip-surface configuration, light can be scattered into both out-of-plane propagating waves and into SPPs that propagate along the surface. For small gaps of a few tens of
nanometers, a large enhancement in the scattering cross section is seen due to strong scattering into SPPs.
We report on experimental realization of different metal-insulator geometries that are used as plasmonic waveguides
guiding electromagnetic radiation along metal-dielectric interfaces via excitation of surface plasmon polaritons (SPPs).
Three configurations are considered: metal strips, symmetric nanowires and nanowire pairs embedded in a dielectric, and
metal V-shaped grooves. Planar plasmonic waveguides based on nm-thin and μm-wide gold strips embedded in a
polymer that support propagation of long-range SPPs are shown to constitute an alternative for integrated optical
circuits. Using uniform and thickness-modulated gold strips different waveguide components including reflecting
gratings can be realized. For applications where polarization is random or changing, metal nanowire waveguides are
shown to be suitable candidates for efficient guiding of arbitrary polarized light. Plasmonic waveguides based on metal
V-grooves that offer subwavelength confinement are also considered. We focus on recent advances in manufacturing of
nanostructured metal strips and metal V-grooves using combined UV, electron-beam and nanoimprint lithography.
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